I think, without an LED, the circuit will be incomplete. It is used as visual indicator in most of the circuits to confirm the working of a particular section. It is a simple diode with light emitting property and hence the name Light Emitting Diode. Its encapsulation has two electrode posts with a chip. When the chip is energized, it emits photons in the form of visible light. The colour of the light depends on the material in the chip. LED works in very low voltage as low as 1 volt. But the new generation White and Blue LEDs require minimum 3 volts to get sufficient brightness. Introduction of the white LED, created a White revolution in the field of LED lighting. Due to its low current consumption, modern lighting system is gradually changing to LED lighting. In addition to White LED, various other types like, Pink LED, Bicolour LED, Multi colour LED etc are also in use . Infrared LED emits light in the Infrared region and hence invisible to eye.
The chip of the LED is the Gallium compound which emits the photons when the p-n junction is forward biased. Brightness is an important aspect of the LED. Human eye has maximum sensitivity to light near 550 nm region of yellow – green part of the visible spectrum. That is why a Green LED appears brighter than a Red LED even though both use same current.
The important parameters of LED responsible for its performance are
1. Luminous flux
Indicates the light energy radiating from the LED. It is measured in terms of Lumen (lm) or Milli lumen (mlm)
2. Luminous intensity
The luminous flux covering a large area is the luminous intensity. It is measured as Candela (cd) or milli candela (mcd) Brightness of LED is directly related to its luminous intensity.
3. Luminous efficacy
It is the emitted light energy relative to the input power. It is measured in terms of lumen per watt (lm w).
Forward current, forward voltage, Viewing angle and Speed of response are the factors affecting the brightness and performance of LEDs. Forward current ( IF ) is the current flowing through the LED when it is forward biased and it should be restricted to 10 to 30 milli amperes other wise LED will be destroyed.
Viewing angle is the off – axis angle at which the luminous intensity fall to half its axial value. This is why LED shows more brightness in full on condition. High bright LEDshave narrow viewing angle so that light is focused into a beam.
Forward voltage (Vf) is the voltage drop across the LED when it conducts. The forward voltage drop range from 1.8 V to 2.6 Volts in ordinary LEDs but in Blue and White it will go up to 5 volts. Speed of response represents how fast an LED is switched on and off. This is an important factor if LEDs are used in communication systems. Forward voltage drop of Typical LEDs is shown below.
LED has a semiconductor chip placed in its centre. The semiconductor consists of two regions namely a P region that has positive charge carriers and an N region with negative charge carriers. There are three layers in the chip. An active photon generating material is sandwiched between the P and N type materials so that photons will be generated when the electrons and holes combines. That is when a potential difference is applied between the P Theand N materials through the LED terminals, holes from the P layer and electrons from the N layer move towards the active material where they combine to produce the light though the phenomenon of Electroluminescence.
The color of photon emission depends on the chemical doped in the active material. Chemicals like Gallium, Arsenic and Phosphor are used to give color to the light emission. Some of the chemicals used in LEDs are
1. Infrared LED- Aluminium gallium arsenide
2.Red LED – Aluminium gallium arsenide, Gallium arsenide phosphide, Gallium phosphide
3. Green LED – Aluminium gallium phosphide, Gallium nitride
4. Yellow LED – Aluminium gallium phosphide, Gallium arsenide phosphide, Gallium phosphide
5. Orange LED – Aluminium gallium indium phosphide
6. Blue LED – Indium gallium nitride, Silicon carbide, Sapphire, Zinc selenide
7. White LED – Gallium nitride based indium gallium nitride
8. Ultraviolet LED – Indium gallium nitride, Aluminium gallium nitride.
Energy level of an LED
The amount of energy in the light generated from an LED depends on the voltage applied to the LED and the electric charge of the electrons present in the LED. The electric charge of an electron is q=
-1.6×10-19 joule. The forward voltage of LED range from 1.8 to 3.6 volts in different types. Therefore the energy of emission can be calculated using the formula E=qV. q is the electric charge of electron and V is the forward voltage drop of LED.
Wattage of an LED
LED is a current dependent device. Minimum 20 mA current is required to get sufficient brightness. If excess current is flowing through the LED, its semiconductor heats up and gradually deteriorate. This leads to poor performance and finally LED will be destroyed. Wattage of the LED is the forward voltage multiplied by the forward current. In high current LEDs, forward current can go up to 350 mA. In these devices the wattage depends on the forward voltage drop ranging from 1.8 volts to 4 volts. Therefore an average of 1 watt is found in high current LEDs.
Light from the LED depends on the material used and the amount of current flowing through it. Luminous intensity of LED depends on the current flowing through it. Typical LED requires minimum 20 mA current to give sufficient brightness. Some semiconductor material is affected by temperature. High power Red and Orange LEDs suffer light loss at high temperature. The light output and viewing angle of LED depends on the semiconductor as well as the epoxy casing of LED. LED with high luminous intensity will have narrow viewing angle because light rays will concentrate into a beam. Diffused LED will scatter light so that intensity will be less.
LEDs have more life when compared to ordinary bulb. The life of LED depends on the physical parameters and operating current. When the light from the LED reduces to half, it indicates its degradation. High quality LEDs can operate more than 1 lakh hours but ordinary LEDs will not live more than 50,000 hours.
Now going to the practical applications of LED. LED is always connected to the power supply through a series resistor. This resistor is called as” Ballast resistor” which protects LED from damage due to excess current. It regulates the forward current to the LED to a safer limit and protects it from burning. Value of the resistor determines the forward current and hence the brightness of LED.
The simple equation Vs – Vf / If is used to select the resistor value. Vs represent input voltage of the circuit, Vf the forward voltage drop of LED and If, the allowable current through the LED. The resulting value will be in Ohms. It is better to restrict the current to a safer limit of 20 m
The table given below helps you to select the series resistor for the LED without going for the calculation.
You should always remember the following while using LED
- LED is a polarized device so it should be connected in the correct polarity. The long lead is positive in most cases but there may be difference. In some LED, both the leads are same and in some other type, the long lead is negative. So check it before connecting. Keep a 3 volt lithium button cell in hand to test the LED.
- Always use the series resistor during LED testing and assembling. It is better to use 1K resistor. Even though the light output reduces, it will increase the life of the LED and also conserve energy especially in battery operated devices.
- Don’t use White or Blue LED as indicators. They consume 3 volts from the circuit and the light is not healthy for the eyes.
- If LED is used in AC as indicator, use a 56K ½ watt resistor. Placing a 1N 4007 diode in series will make the light steady.
- In battery operated devices, it is better to avoid LED to conserve power. If you want it, use a push switch for the LED, so that the power can checked if needed.
Please wait for the next article The Trick of White LED
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